It is known how to employ liquid/liquid or liquid/gaseous membrane modules for material exchange systems, and tube modules which work with hollow fibers or capillary tubes have proven to be especially good. The tubes in this case are secured in liquid and gas-tight manner, preferably cast, in two face plates of a housing. A fluid involved in the material exchange, for example a gas, flows through the tubes. A bundle of fibers is arranged in a housing through which another fluid flows, for example a liquid. The efficiency of the material exchange is influenced by the flow relations, among other things. The flow can approach the tubes in the axial or radial direction.
Such modules are used, for example, for ultra-filtration and microfiltration, as well as for degasification, e.g., of organic liquids, and, more recently, for cleaning of exhaust gas. In exhaust gas cleaning and similar applications, the gas being cleaned flows through the tube membranes, for example, and a liquid flows around the outside of the tubes, said liquid having absorptive properties and being able to bind pollutants.
To improve the efficiency of the material exchange it has proven to be advantageous to employ tube membranes of PTFE, since these possess both good chemical and thermal stability and, in particular, a low surface tension and, accordingly, very good hydrophobic properties.
Known PTFE tubes are made by paste extrusion. The wall thicknesses achievable are limited by this method. It is only possible to orient the material in the lengthwise direction. Furthermore, the method is expensive and the tubes are therefore costly.
A method is known from U.S. Pat. No. 4,791,966 for production of tubes, in which a flat band of microporous, oriented PTFE, thickened in the middle in cross section, is wound in a spiral about a wire, sintered, and then pulled away from the wire. This method has a number of advantages over the first-mentioned technique. In particular, the wall thickness of the tube membranes can be varied in broad limits. Also, a material with a particular microporosity can be used from the very beginning. The drawback is, in particular, the awkward handling of the tubes when putting together the module, as well as the still quite expensive and costly fabrication of the tubes.